An ephemeral meandering river system: Sediment dispersal processes in the Río Colorado, Southern Altiplano Plateau, Bolivia
Li, Jiaguang; Luthi, Stefan M.; Donselaar, Marinus E.; Weltje, Gert Jan; Prins, Maarten A.; Bloemsma, Menno R.
published: Sep 1, 2015
The Río Colorado meandering river system feeds the Salar de Uyuni, the World's largest salt pan in the southern Altiplano plateau (Bolivia). It is characterized by ephemerality due to the high aridity of the region, and a downstream decrease of river bankfull width and depth in the river terminus. Dryland meandering river systems in high-altitude regions have rarely been reported. We investigated the active processes in this system from the alluvial fan to the lower coastal plain. On-site surveys combined with high-precision GPS and high-resolution satellite imagery show that Río Colorado area is typified by generally fine sediment and a low gradient in the coastal plain. In the alluvial fan, where channel meandering is weak but still recognizable, the deposits are characterized by fining-upward sequences with gravel-prone sediments at the bottom, paleosols in the middle and clay-dominated sediments at the top. In the coastal plain the channel deposits consist of coarse to fine sand with sparse basal gravel, while the river banks and thus much of the floodplain consist of clay and silt in the upper coastal plain and very fine sand, silt and clay with salt deposits in the lower coastal plain. Grain-size distribution analysis show a linear decrease in bed load sediments downstream and a concurrent increase in suspended load sediments. These longitudinal variations in the proportions of sediment load are found to be consistent with changes in the topographic slope. The river gradient shows six intervals with downstream decreasing but internally quite constant slopes and sediment composition. Characteristic geomorphologic features are found to be head-cutting channels, chute channels, avulsions and crevasse splays. They illustrate the processes of erosion and deposition in this low-gradient river system and help understand sediment dispersal and their preservation potentials.